At first I thought I was just unlucky but 4/5 portals I've created have put me either in the air or right above lava lakes with no supporting structures generating.

Is that normal or am I doing something wrong?

This is my average experience

If the corresponding nether position is close to a lava lake then it has a high probability to generate above the lava lake. It's normal.

I'm going to do some comparisions from the same block on vanilla and modded and test what happens. I can't recall ever landing in such a situation in vanilla

So I'm using -857414580999580583 as the seed for the tests with
This modded setup:
Minecraft 1.16.3
Fabric Loader: 0.10.0+build.208
Phosphor: 0.6.0+build.7
Immersive Portals 0.36

And pure vanilla 1.16.3.

All portals are full portals with corners, 4 obsidian on the bottom, 5 on the sides, making a 2x3 portal.
All coordinates correspond to the bottom right corner while facing south, and entering the portal facing south.

Here are my results:

Test1:

Overworld coords:
222, 97, 1164

Modded nether exit coords: 37, 32, 140

Vanilla nether exit coords: 15, 73, 140

Test 2

Overworld coords: 313, 113, 1790

Modded nether exit: 34, 58, 218

Vanilla nether exit: 40, 99, 215

I could keep on going but this is the majority of the cases. In conclusion, I don't think the valid portal location detection works the way it does with vanilla.

Maybe it could run the vanilla check then use that as the portal location?

That would be awesome. Right now I have to decide whether to include it in a server and see this as a hard challenge or jus go with vanilla. :/

The vanilla algorithm only generated 2x3 simple nether portal so that cannot be used. I changed the portal generation to decrease the possibility of generating portal above lava lake. Will be in the next version

Sounds good! I don't mind a challenge once every so often but lava lake/mid air generation should definitely be the minority of cases.